JERUSALEM — One of Prime Minister Benjamin Netanyahu’s closest and longest-serving aides appeared ready to incriminate him on Wednesday after agreeing overnight to become a government witness, the latest twist in a spiraling graft scandal that seemed to dim Mr. Netanyahu’s legal and political chances of survival almost by the hour.

The fast-moving police inquiry into whether Mr. Netanyahu, already battling separate bribery allegations, had provided official favors to Israel’s largest telecommunications company, Bezeq, in exchange for fawning coverage on the company’s online news site prompted one member of the prime minister’s party to ask him to step aside and opposition politicians to call for early elections.

Mr. Netanyahu, who insists he has done nothing wrong, has faced corruption allegations periodically almost since first becoming prime minister in 1996. But the latest — with its suggestion of political payoffs to a company that bills ordinary Israeli voters every month — could prove the most damning. And as the revelations mounted, one on top of another like a tottering tower, Israelis expressed increasing doubt about Mr. Netanyahu’s ability to maintain his grip on power.

“It was like watching a police car chase in pursuit of a robber on one of America’s endless highways,” Sima Kadmon, a columnist, wrote in Wednesday’s Yedioth Ahronoth of the flurry of events of the day before. “Riveting hours of dramatic and fateful revelations that are going to change not only the life of the man behind the wheel, but the face of our country.”

The new state’s witness, Shlomo Filber — who goes by “Momo” — was director general of the Communications Ministry from 2015 until 2017, answering directly to Mr. Netanyahu, who at the time also held the portfolio of communications minister. The ministry ruled or weighed in on a number of key regulatory decisions that provided enormous financial benefits to Bezeq and its controlling shareholder, Shaul Elovitch.

Mr. Filber, who was suspended from his post a few months ago as regulators from the Israel Securities Authority closed in, was arrested this week along with a number of other high-profile friends and confidants of Mr. Netanyahu, including Mr. Elovitch, members of his family and other senior Bezeq executives.

Mr. Filber’s testimony promised to connect the two main arms of the Bezeq affair: the Communications Ministry’s dealings with the company and the decisions taken by editors at Walla, its online news subsidiary, according to a person with intimate knowledge of the inquiry who was not authorized to speak about it publicly. In exchange, prosecutors agreed not to seek a prison sentence for Mr. Filber.

Such state’s witness agreements in Israel usually involve a criminal suspect giving up a bigger fish. Mr. Filber’s direct superior was Mr. Netanyahu.

Mr. Filber was expected to tell the police that in his role at the ministry he was carrying out explicit and detailed instructions from Mr. Netanyahu, according to Israel’s Channel 2 evening “News” program, and that he came to realize he was being exploited.

Through a spokesman, Mr. Netanyahu responded, “It never happened.”

“The main question is, what does he know?” said Oren Gazal-Ayal, dean of the University of Haifa law faculty and a criminal-law specialist. “According to suspicion, Filber was the one offering Bezeq all the benefits. He changed the policies of the government from the minute he became director-general of the ministry. If Netanyahu changed the regulations to get good coverage, and you can prove that, that’s definitely a criminal offense.”

In his first response to Mr. Filber’s agreement with prosecutors, Mr. Netanyahu projected bravado. He posted part of a biblical verse, Exodus 1:12, on his Facebook page — “But the more they were oppressed, the more they multiplied and spread” — referring to the Egyptians’ dread of the enslaved Israelites gaining strength. With it, he included a chart from a survey by his own pollster, Geocartography, showing a spike in support for his conservative Likud Party.

The cases involving Mr. Netanyahu or his close associates have been piling up at a dizzying pace. Along with the developments in the Bezeq matter, which the police call Case 4000, were new suggestions that another of Mr. Netanyahu’s aides tried to bribe a judge to quash a criminal case involving his wife.

Mr. Netanyahu has described both sets of allegations as “baseless” and “delusional” and has not yet been named a suspect or called for questioning in either case.

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Prime Minister Benjamin Netanyahu of Israel and his wife Sara in Ashkelon, Israel, on Tuesday.CreditAmir Cohen/Reuters

But he is already deeply embroiled in two other corruption cases. One, known as Case 1000, involves accusations of illicit gifts for favors. In the other, Case 2000, he is accused of back-room dealing with the publisher of Yedioth Ahronoth, a major Israeli newspaper, also for more favorable coverage. The police recommended last week that Mr. Netanyahu be prosecuted on charges of bribery, fraud and breach of trust. Mr. Netanyahu denies any wrongdoing.

Any actual indictment may be months away, pending a hearing with Mr. Netanyahu’s lawyers and a decision by the attorney general.

Adding to the prime minister’s woes, however, Mr. Filber is not the first former ally to have turned against Mr. Netanyahu. Many analysts predicted as early as August that Mr. Netanyahu was doomed after the police signed a state’s witness deal with Ari Harow, his former chief of staff and once one of his closest confidants, in Cases 1000 and 2000.

By Wednesday morning, the first hairline crack in Mr. Netanyahu’s coalition had become visible, as a fellow member of Likud, Oren Hazan, called on Mr. Netanyahu to step aside. It was unclear whether other lawmakers would follow: Mr. Hazan is better known for headline-grabbing stunts than for influencing his colleagues.

Mr. Filber, on the other hand, is a consummate political insider. He has worked for Likud since the mid-1990s and ran the party’s election campaign in 2015.

His decision to become a government witness was treated as a bombshell: Yedioth Ahronoth’s front page screamed, “Earthquake.”

“Filber is the closest and most intimate covert operations officer that Netanyahu has had in generations,” the Israeli journalist Ben Caspit, author of a recent biography, “The Netanyahu Years,” wrote in Maariv. “As early the 1990s he was the secret researcher deployed by the campaign for all tasks that were best kept under wraps. Always in the shadows, always loyal, efficient, secretive and ideological, Bibi knew that he could count on Momo.”

Mr. Filber’s lawyers, Dror Arad-Ayalon and Yovel Nachmani, did not immediately comment after news broke of his deal with prosecutors. Nor did the police or Justice Ministry immediately put out any announcement detailing the arrangement.

Mr. Filber, 54, who has described himself as a strategic and business consultant, sits on the board of the state-owned Israel Military Industries and served from 2003 to 2009 as the board secretary of the state-owned Israel Railways and as its assets and business development manager.

Before that, he headed Mr. Netanyahu’s headquarters in years when Mr. Netanyahu was both in and out of office. From 1996 until 2001, he held leadership positions in the Yesha Council, the umbrella organization representing Israeli settlers in the West Bank and, at the time, in Gaza.

The son of a rabbi, Yaakov Filber, who was a leader in the religious Zionist movement and the founder of a yeshiva and research center, Mr. Filber lived in the West Bank settlement of Psagot before moving to the central city of Petah Tikva, east of Tel Aviv.

For all his years with Mr. Netanyahu, Mr. Filber appears to go back even further with the head of the Israeli police, Roni Alsheich, who has overseen the sprawling corruption investigations.

In a Twitter post congratulating Mr. Alsheich on his appointment as commissioner in September 2015, Mr. Filber referred to their paths having crossed over three decades, in a religious seminary, in the army and in the settlements of the West Bank.

“Good luck Roni Alsheich,” Mr. Filber wrote. “There’s nobody more suited than him. We have known each other for 30 years — from the benches of ‘Merkaz Harav,’ paratrooper battles in west Beirut, settlement in Binyamin and until today.”

Irit Pazner Garshowitz contributed reporting.

A version of this article appears in print on , on Page A4 of the New York edition with the headline: Netanyahu Case Builds as a Top Aide Turns Witness. Order Reprints | Today’s Paper | Subscribe